As one of radio communication schemes, a duplex (full duplex) scheme with a transmit frequency fT and a receive frequency fR is known. A communication apparatus used for this type of a scheme modulates the carrier by a baseband signal to be transmitted (transmit frequency fT), and extracts the baseband signal by demodulating the received wave (receive frequency fR). The transmission uses not only a direct modulation scheme that modulates the carrier directly by the baseband signal, but also an indirect modulation scheme that converts the baseband signal to an intermediate frequency signal with a frequency fTIF, followed by frequency conversion to generate a transmit wave with the frequency fT. On the other hand, although the reception usually employs the so-called superheterodyne scheme, an indirect demodulation scheme using the intermediate frequency (frequency fRIF), it sometimes employs a direct demodulation scheme that demodulates the incoming wave directly at the carrier frequency.
Even when this type of communication apparatus employs the direct modulation scheme, it is unusual to oscillate the signal with the transmit frequency fT directly by a VCO (voltage-controlled oscillator). The signal with the frequency fT is usually generated by oscillating two frequencies independently using two VCOs, followed by mixing them with a mixer. This is because when generating the signal with the frequency fT directly by a single VCO, the unmodulated signal component (carrier component) of the same frequency leaks out of the circuit, causing communication interference. Thus, to reduce the carrier leakage, the configuration using the two VCOs is employed.
On the other hand, at the receiving side, the superheterodyne scheme converts the incoming wave with the receive frequency fR to the intermediate frequency (frequency fRIF) with a mixer using a first VCO, removes undesired waves with a filter, and then extracts the baseband signal using a second VCO (oscillation frequency fRIF). In the direct demodulation scheme also, the local signal (with the same frequency as the incoming wave carrier frequency fR) used for demodulation is usually generated by oscillating two frequencies by two independent VCOs, followed by mixing them to generate the signal with the frequency fR. This is because generating the frequency fR by a single VCO can arise various problems owing to the circulation of the signal within the apparatus.
Thus, regardless of the indirect or direct modulation and demodulation scheme, both the transmitting and receiving sides require two VCOs each. An actual communication apparatus, however, usually uses three VCOs by sharing one of the four VCOs in order to simplify the apparatus and to increase efficiency of the frequency channel selection operation.
FIG. 1 shows a conventional communication apparatus using the indirect modulation scheme, and FIG. 2A shows a conventional communication apparatus employing the direct modulation scheme. As clearly seen from these figures, they use three VCOs (VCO1-VCO3) each. FIG. 2B shows a configuration in which a receiving system uses the direct scheme as well.
It is assumed in FIG. 1 and FIGS. 2A and 2B that the transmit frequency fT is higher than the receive frequency fR, and the frequency difference between them is fDD. Assume further that the frequency of the shared VCO 1 is fCH, and that the oscillation frequencies of the VCO 2 and VCO 3 used for the transmission and reception are fTIF and fRIF, respectively, regardless of the direct or indirect modulation and demodulation. Then, the following expressions hold.fCH=fT±fTIF fCH=fR±fRIF wherefT=fR+fDD 
To select the frequencies fCH, fTIF and fRIF satisfying the foregoing expressions, it is necessary for an actual system to prevent spurious components, which are caused by combinations of harmonics of the oscillation frequencies, from overlapping on the transmission band or reception band. Furthermore, as with the superheterodyne scheme, a desired combination of the frequencies fCH, fTIF and fRIF satisfying the foregoing expressions is usually selected considering practically important conditions such as selecting the frequency fRIF that will facilitate implementing a filter for the frequency fRIF.
As for the communication apparatus including a lot of channels as its transmission channels and reception channels, the oscillation frequency fCH is made variable for the channel selection, and the frequencies fTIF and fRIF are fixed to simplify the apparatus.
The conventional system configurations as shown in FIG. 1 and FIGS. 2A and 2B have an advantage of having a high degree of flexibility in selecting the fCH, fTIF and fRIF, and of being able to handle the spurious problem and the like with ease, although they require three VCOs.
However, requiring three VCOs is undesirable for a mobile communication apparatus that demands to be compact and lightweight, low current consumption and low cost.
In view of this, a configuration as shown in FIG. 3A is conceived to implement a communication scheme requiring only two VCOs and having a frequency difference fDD (Hz) between the transmit frequency and receive frequency, for example.
Incidentally, although the indirect scheme is used for the transmission and reception for the sake of description in the accompanying drawings, the present invention is also applicable in exactly the same manner to the case where one of the transmission and reception or both of them use the direct scheme as described above in connection with FIG. 1 and FIGS. 2A and 2B.
Assume that the oscillation frequencies of the two VCOs are fVCO1 and fVCO2, then the following equations hold.fVCO1=fCH=fR+(2×fDD)fVCO2=2×fDD
This means that the frequencies fTIF=fDD and fRIF=2×fDD are selected. However, the configuration as shown in FIGS. 3A-3C has a problem in that a certain transmission signal has a spurious component overlapping on the reception band. Specifically, one of the spurious components of the transmission signal consisting of a combination of the fundamental fCH and the second harmonic fTIF falls exactly on the receive frequency as given by the following equation.fCH−(2×fTIF)=(fR+(2×fDD))−2×fDD=fR
Thus, since the spurious component of the transmission signal exactly agrees with the receive frequency, circulation occurs from a transmitting side circuit to a receiving side circuit within the apparatus, thereby causing extra noise. In addition, this also presents a problem of sending an interference wave to other communication apparatuses.
In brief, although the example of FIGS. 3A-3C has an advantage of being able to simplify the configuration by using the two VCOs and one frequency divider, it has an unavoidable problem concerning the frequencies.
FIGS. 4A-4C show an example that selects the frequencies such that fVCO2=4×fDD holds. In this case, to establish the relationships fTIF=fDD and fRIF=2×fDD as in FIGS. 3A-3C, a transmitting side mixer is supplied with the output of a divide-by-4 circuit (frequency fDD), and a receiving side mixer is supplied with the output of a divide-by-2 circuit (frequency 2×fDD). However, the example also has a problem in that one of the spurious components of the transmission signal exactly agrees with the receive frequency as described above in connection with FIGS. 3A-3C.
FIGS. 5A-5C show an example that selects the frequency fVCO2 satisfying the relationship fVCO2=4×fDD as in FIGS. 4A-4C. However, to establish the relationships fTIF=2×fDD and fRIF=fDD, the transmitting side mixer is supplied with the output of the divide-by-2 circuit (frequency 2×fDD), and the receiving side mixer is supplied with the output of the divide-by-4 circuit (frequency fDD). In this example, the problem in that the spurious component of the transmission signal exactly agrees with the receive frequency does not occur.
Furthermore, FIGS. 6A-6C show an example from which the two frequency dividers of FIGS. 4A-4C or FIGS. 5A-5C are removed. In this example, the VCO 1 and VCO 2 have the following oscillation frequencies.fVCO1=fCH=fR+(½×fDD)fVCO2=½×fDD
Accordingly, the relationships fRIF=fTIF=½×fDD hold. Thus, it is possible to make the frequency fT higher than fR by fDD by selecting the fT=fCH+fTIF and fR=fCH−fRIF side mixing components.
In this case, however, the spurious problem is more serious, resulting in that the sideband unnecessary for the transmission overlaps on the reception band strongly.
As described above, although various apparatuses are conceivable which generate the transmit frequency fT and receive frequency fR using only two VCOS, the configurations other than the configuration of FIGS. 5A-5C have a transmission spectrum that overlaps on the reception band, thereby offering a problem of little practical utility.
FIGS. 5A-5C show the only one configuration serving practical use without the transmission spurious problem among the simple configurations briefed above. However, the system whose acceptable frequencies fT and fR (and hence fDD) are determined, has no other options even if the frequency structure has a problem. For example, when it is difficult to implement a compact and practical superheterodyne IF filter at the frequency fRIF (=fDD), it is fatal for the two-VCO configuration not to have other alternative.
In view of the foregoing problems, an object of the present invention is to provide a communication apparatus capable of generating the transmission signal and reception signal using two oscillators, and of preventing a harmful spurious component from being generated even with a configuration other than that of FIGS. 5A-5C.
Generally speaking, greater use of complicated frequency conversions can enable other configurations with two VCOs. However, complicated frequency conversions will make the system worse than the systems with three VCOs in terms of compactness, light weight, low power consumption and low cost, which will make the present invention nonsense.
Therefore another object of the present invention is to provide a communication apparatus whose frequency converters, which are necessary to achieve the foregoing object, have a simple configuration.